The basic idea of a firing system using pulverized coal is to use the furnace for the combustion of solid fuels. Coal is ground to the size of a fine grain, mixed with air and burned in the flue gas flow. High velocity airflow is required to move the coal through the pulverizer. Coal contains mineral matter including rocks and aggregate, which is converted to ash during combustion. The ash is removed as bottom ash and fly ash. The bottom ash is removed at the furnace bottom. Coal that has been pulverized into a fine powder stems will burn almost as easily and efficiently as a gas. Pieces of coal are crushed by balls or cylindrical rollers that move between two tracks or races. The raw coal is then fed into the pulverizer along with air heated to about 350-450 degrees F. from the boiler. As the coal is crushed by the rolling action, the hot air dries it and blows the usable fine coal powder out to be used as fuel. The powdered coal from the pulverizer is directly blown to a burner in the boiler. Due to the nature of the pulverizer that uses heated high velocity air flow and which pulverizes coal containing rocks and other foreign materials, pulverizers are typically plagued by wear problems due to erosion of the parts in the pulverizer. Required repair and replacement of worn parts can involve prolonged shut down of the pulverizers and loss of production for several days.
As shown in FIG. 1, the overall design of a vertical spindle coal pulverizer is generally that of a prior art Babcock and Wilcox pulverizer model MPS-89. Raw coal is fed into the top of the pulverizer 10 through the raw coal pipe 11 and descends to the grinding ring 12 where it is broken by grinding wheels 13. The grinding ring 12 has a base or yoke 15 turned by a motor shaft 16, resulting in the rotation of the grinding wheels 13 on the grinding ring 12. The pulverized coal is centrifugally thrust by the rapid turning of the grinding ring across air channels 18 of throat ring 19. Throat ring 19 concentrically surrounds the coal grinding assembly such that all of the pulverized coal passes over the throat ring 19 and the air channels 18. The grinding table includes a ring seat that may be either a single-piece casting or a weldment, and contains a plurality of grinding segments that form a circular track in which the heavy grinding rolls contact and crush the coal. The inner wall of the pulverizer is typically steel or cast steel and has an inner layer of ceramic tile on it above the rotating throat assembly to provide wear resistance for the wall surface.
Forced air is supplied through air inlet 21 to and through the air channels 18 of the throat ring. Primary air is pre-heated by a primary heater before the air enters the mill. The primary air inlet temperature varies for different coal types, moisture content, etc. Typically, the primary air inlet temperature ranges from about 250 F-475 F. The primary air inlet temperature is set in order to maintain a consistent outlet temperature at which the air/fuel mixture is sent to the burners. The classifier 22 functions to segregate the coal which is fine enough to be burned from that which must be returned to the pulverizer because it is still too large. The relatively fine coal is carried with the air to the boiler through discharge turret 24 and burner pipes 11A, partially controlled by burner pipe valves 25, while the recycled larger particles fall back to the grinding ring 12 by way of classifier cone 26. Relatively dense mineral particles hopefully find their way to pyrites box 14.
U.S. Pat. Nos. 5,549,251 and 5,908,167, which are assigned to Techinomics, Inc., disclose rotating throat assemblies for coal pulverizers in which primary air performs four functions in the pulverizer: (1) drying of the coal in the pulverizer, (2) maintaining a fluidized bed of coal, which circulates coal into the path of the grinding elements, (3) transporting the coal particles from the fluidized bed into the classifier assembly, where large particles are separated for return to the grinding elements, and (4) transporting suitably pulverized coal particles out of the classifier to the burners. U.S. Pat. No. 5,549,251 discloses an cylindrical air seal (unnumbered and not explained) on the wall of the pulverizer adjacent the upper edge of the rotatable throat assembly in the pulverizer and a support ring 30 on the wall of the pulverizer adjacent the lower edge of the rotatable throat assembly. Techinomics' rotating throat assemblies are particularly efficient in the use of the primary by providing higher air velocities without increasing the air flow mass. There is a very wide range of cfm PA (primary air) feeding the mill. This is dependent on the size/capacity of the mill in tons, mill loading, coal type & moisture, altitude, etc.
Babcock & Wilcox's U.S. Pat. No. 5,340,041 discloses a replaceable passage arrangement for a pulverizer having a fixed housing with a central axis. The replaceable passage design consists of a passage segment, which comprises a number of individual parts, which are attached or welded together. Each passage segment has an inner rail and outer rail, which are spaced parallel from each other and secured in position by a plurality of ribs. The passage segment is mounted with fasteners and/or welding, to the grinding table. A replaceable ledge cover assembly is secured to the wall of the pulverizer and forms an inlet cone around the axis of the pulverizer for the flow of primary air.
Separation of the smaller and larger coal particles and recycling of the larger ones is common to most, if not all, pulverizer designs. The task is complicated, however, by the presence of relatively dense non-combustible materials, i.e. rock, which is incidentally introduced as part of the coal feed. Where the machine functions to recycle such non-combustible materials rather than separating them out, the inefficiency is manifest. The machine not only expends unnecessary energy on recirculating and regrinding a material of zero fuel value, but does so at the cost of considerable wear. If somehow the rock particles are not rejected from the coal being processed, they must be reduced in size until they can mix with the coal particles transported to the burners. The presence of rock particles in the fuel stream reduces combustion efficiency and also results in a greater and faster buildup of ash in the combustion chamber, further reducing boiler efficiency by retarding heat transfer from the combustion chamber.
An improved coal pulverizer is needed that will provide high efficiency while minimizing wear of the pulverizer and shorten the time required to replace or repair worn parts in the pulverizer.